1. Field of Invention
The present invention relates to a process for preparing microporous membranes from a blend containing an unsulfonated poly(phenylene sulfide) (PPS) polymer, an amorphous polymer, and optionally a solvent and/or optional non-solvent. Such membranes are useful in the treatment of liquids by the membrane separation processes of ultrafiltration, microfiltration, depth filtration, macrofiltration, membrane distillation, and membrane stripping. The membranes of this invention are also useful as microporous supports for composite liquid and/or gas separation membranes.
2. Description of Related Art
In the past, microporous membranes have been fabricated from polyolefins such as polyethylene and polypropylene. One typical method of preparing such polyolefin membranes is by an extrusion process which involves dissolving the polyolefin in a solvent or a mixture of solvent and non-solvent, extruding the polyolefin/solvent/non-solvent mixture into membranes, and immersing the membranes into a leach bath. Another method of preparing such polyolefin membranes is by a melt-extrusion process which involves extruding the membranes from the molten polyolefin, followed by cold drawing the membranes. However, polyolefins, while inexpensive and easy to process, exhibit relatively low heat distortion temperatures.
Poly(phenylene sulfide) polymers are high performance thermoplastics which possess high glass transition temperatures, high crystalline melting points, high thermal stability, and high solvent resistance. Such properties make poly(phenylene sulfide) polymers useful for membranes employed in liquid separations, particularly membrane separation processes which involve treatment of organic, acidic, or basic liquids at elevated temperatures.
The very properties which make poly(phenylene sulfide) polymers desirable materials for use in applications which require high temperature and/or solvent resistance also render such polymers very difficult to process into membranes, particularly since poly(phenylene sulfide) polymers exhibit relatively low solution viscosities at the high membrane fabrication temperatures, in excess of about 250.degree. C., frequently required to fabricate membranes. The low solution viscosities exhibited by poly(phenylene sulfide) polymers are particularly problematic with extrusion or casting blends containing less than about the 40 weight percent polymer required to produce high flux microporous membranes. Such low solution viscosities also render extrusion of hollow fiber microporous membranes from poly(phenylene sulfide) polymers especially difficult.
Furthermore, poly(phenylene sulfide) polymers are extremely solvent resistant and are therefore considered to be insoluble in all common solvents. Therefore, to form membranes, PPS, for example, is expected to be dissolved in very strong acids such as concentrated sulfuric acid to sulfonate the poly(phenylene sulfide), which renders the sulfonated poly(phenylene sulfide) soluble in common solvents such as dimethylformamide and dimethylacetamide. The problem associated with such a process is that the fabricated membrane comprises not poly(phenylene sulfide), but rather sulfonated poly(phenylene sulfide), which is soluble in common solvents. Thus the high solvent resistance of poly(phenylene sulfide) is lost.
What is needed is a process of preparing microporous membranes from unsulfonated poly(phenylene sulfide) polymers using plasticizers, that is, solvents and optional non-solvents, which do not chemically modify or degrade the unsulfonated poly(phenylene sulfide) polymer during fabrication so that the high strength, temperature resistance, and solvent resistance of the unsulfonated poly(phenylene sulfide) polymer is retained by the fabricated membranes.
What is further needed is a method of increasing the solution viscosities of the poly(phenylene sulfide) polymers, so that membranes can be more easily fabricated at the high temperatures required to fabricate membranes from such polymers, while retaining the high temperature and solvent resistance of the unsulfonated poly(phenylene sulfide) polymer.
What is especially needed is a process for preparing microporous membranes having high flux from unsulfonated poly(phenylene sulfide) polymers.
The membranes of the present invention accomplish these objectives and exhibit excellent solvent and temperature resistance. The membranes also possess high tensile strength. The membranes are useful as microporous membranes for liquid separations such as ultrafiltration, microfiltration, depth filtration, macrofiltration, membrane stripping, and membrane distillation and as microporous supports for composite liquid or gas separation membranes.